Bile acid malabsorption is a condition characterized by an excess of bile acids in the colon, often leading to chronic diarrhoea. Bile acids are steroid acids that are synthesized and conjugated in the liver. From the liver, they are excreted through the biliary tree into the small intestine where they participate in the solubilisation and absorption of dietary lipids and fat-soluble vitamins. When they reach the ileum, bile acids are reabsorbed into the portal circulation and returned to the liver. A small proportion of the secreted bile acids is not reabsorbed in the ileum and reaches the colon. Here, bacterial action results in deconjugation and dehydroxylation of the bile acids, producing the secondary bile acids deoxycholate and lithocholate.
In the colon, bile acids (in particular the dehydroxylated bile acids chenodeoxycholate and deoxycholate) stimulate the secretion of electrolytes and water. This increases the colonic motility and shortens the colonic transit time. If present in excess, bile acids produce diarrhoea with other gastrointestinal symptoms such as bloating, urgency and faecal incontinence. There have been several recent advances in the understanding of this condition of bile salt or bile acid malabsorption, or BAM (Pattni and Walters, Br. Med. Bull. 2009, vol 92, p. 79-93; Islam and Di Baise, Pract. Gastroenterol. 2012, vol. 36(10), p. 32-44). Dependent on the cause of the failure of the distal ileum to absorb bile acids, bile acid malabsorption may be divided into Type 1, Type 2 and Type 3 BAM.
Diarrhoea may also be the result of high concentrations of bile acid in the large intestine following treatment with drugs that increase the production of bile acids and/or influence the reabsorption of bile acids by the small intestine, such as treatment with ileal bile acid absorption (IBAT) inhibitors.
The current treatment of bile acid malabsorption aims at binding excess bile acids in the gastrointestinal tract, beginning in the proximal part of the small bowel, thereby reducing the secretory actions of the bile acids. For this purpose, cholestyramine is commonly used as a bile acid sequestrant. Cholestyramine (or colestyramine; CAS Number 11041-12-6) is a strongly basic anion-exchange resin that is practically insoluble in water and is not absorbed from the gastrointestinal tract. Instead, it absorbs and combines with the bile acids in the intestine to form an insoluble complex. The complex that is formed upon binding of the bile acids to the resin is excreted in the faeces. The resin thereby prevents the normal reabsorption of bile acids through the enterohepatic circulation, leading to an increased conversion of cholesterol to bile acids to replace those removed from reabsorption. This conversion lowers plasma cholesterol concentrations, mainly by lowering of the low-density lipoprotein (LDL)-cholesterol.
Cholestyramine is also used as hypolipidaemic agents in the treatment of hypercholesterolemia, type II hyperlipoproteinaemia and in type 2 diabetes mellitus. It is furthermore used for the relief of diarrhoea associated with ileal resection, Crohn's disease, vagotomy, diabetic vagal neuropathy and radiation, as well as for the treatment of pruritus in patients with cholestasis.
In the current treatment of hyperlipidaemias and diarrhoea, the oral cholestyramine dose is 12 to 24 g daily, administered as a single dose or in up to 4 divided doses. In the treatment of pruritus, doses of 4 to 8 g are usually sufficient. Cholestyramine may be introduced gradually over 3 to 4 weeks to minimize the gastrointestinal effects. The most common side-effect is constipation, while other gastrointestinal side-effects are bloating, abdominal discomfort and pain, heartburn, flatulence and nausea/vomiting. There is an increased risk for gallstones due to increased cholesterol concentration in bile. High doses may cause steatorrhoea by interference with the gastrointestinal absorption of fats and concomitant decreased absorption of fat-soluble vitamins. Chronic administration may result in an increased bleeding tendency due to hypoprothrombinaemia associated with vitamin K deficiency or may lead to osteoporosis due to impaired calcium and vitamin D absorption. There are also occasional reports of skin rashes and pruritus of the tongue, skin and perianal region. Due to poor taste and texture and the various side effects, >50% of patients discontinue therapy within 12 months.
Another drawback with the current treatment using cholestyramine is that this agent reduces the absorption of other drugs administered concomitantly, such as oestrogens, thiazide diuretics, digoxin and related alkaloids, loperamide, phenylbutazone, barbiturates, thyroid hormones, warfarin and some antibiotics. It is therefore recommended that other drugs should be taken at least 1 hour before or 4 to 6 hours after the administration of cholestyramine. Dose adjustments of concomitantly taken drugs may still be necessary to perform.
In view of these side effects, it would be desirable if cholestyramine could be formulated as a colon release formulation, i.e. for release of the cholestyramine in the proximal part of the colon. Such a formulation may require a lower dose of cholestyramine and should have better properties regarding texture and taste, and may therefore be better tolerated by the patients. More importantly, colonic release of cholestyramine should be devoid of producing interactions with other drugs and should not induce risks for malabsorption of fat and fat-soluble vitamins, while still binding bile acids in order to reduce the increased colonic secretion and motility. For reasons of patient compliance, it would furthermore be desirable if the number of pills to be taken could be kept as low as possible. Each pill should therefore contain as much cholestyramine as possible.
EP 1273307 discloses preparations for preventing bile acid diarrhoea, comprising a bile acid adsorbent coated with a polymer so as to allow the release of the bile acid adsorbent around an area from the lower part of the small intestine to the cecum. It is shown that cholestyramine granules coated with HPMCAS-HF or ethyl cellulose displayed extensive swelling and bursting under conditions simulating the gastric environment.
Jacobsen et al. (Br. Med. J. 1985, vol. 290, p. 1315-1318) describe a study wherein patients who had undergone ileal resection were administered 500 mg cholestyramine tablets coated with cellulose acetate phthalate (12 tablets daily). In five of the 14 patients in this study, the tablets did not disintegrate in the desired place.
Despite progress made in this area, there still is a need for further improved cholestyramine formulations. In particular, there is a need for small cholestyramine particles that have a high cholestyramine content and are stable during the coating process.